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Flying high

Balloon-borne project teams U.S. and French scientists in atmospheric research

 

When an international team of scientists lands at McMurdo Station in early August 2010, it will be dark and cold as only an Antarctic winter can be.

And that’s a good thing if you’re interested in learning about what’s happening in the atmosphere above the continent at one of the harshest times of the year.

The mostly French and American researchers will launch as many as 18 long-duration balloons to float in the atmosphere at about 20 kilometers altitude. The “launch pad” will be from a field camp on the sea ice in front of the U.S. Antarctic Program’s biggest research station.

These super-pressure balloons, capable of maintaining a level altitude, will carry a host of different instruments for measuring everything from basic atmospheric properties like temperature and humidity, including their profiles to the ice surface, to the processes involved in ozone depletion over Antarctica.

The project, a holdover from the International Polar Year campaign that officially ended in 2009, is called Concordiasi.

“Concordiasi is motivated by the urgent need to reduce uncertainties in diverse, but complementary, fields in Antarctic science,” said Florence Rabier, with Centre National de Recherches Météorologiques, a joint research center by Météo-France, the French national meteorological service, and CNRS, the French Scientific National Center.

“The project involves many instruments and institutes, which is always an organizational challenge,” she said via e-mail. “However, it is also a strength, as many scientists from different backgrounds have an interest in the field campaign, which can only lead to interesting scientific results.”

Clouding the issue

One of the scientists is Terry Deshler, whose team from the University of Wyoming has been monitoring the annual ozone hole for almost 25 years. Deshler and his group usually arrive every August before the main Antarctic field season begins in October. That’s to catch the ozone hole as it begins to form and strengthen, fueled by the increasing spring sunlight, before finally reaching its full extent in November.

Scientists have learned much about the chemistry and physics behind ozone depletion over the years — but not everything. One of the missing pieces involves the formation of particles in polar stratospheric clouds (PSCs). The PSCs in the stratosphere provide the “platform” on which the chemical reactions occur that destroy ozone. [See Ozone Refresher for more detail.]

In particular, Deshler’s team wants to learn more about the formation of nitric acid tri-hydrate (NAT), one of three types of particles that form in PSCs. The three types of PSC particles nucleate at slightly different temperatures around the minus 80 degrees centigrade barrier. Knowing the conditions under which the NAT particles form could help with ozone depletion forecasting around the world.

“If you want to model polar ozone loss, you need to decide which temperature you’re going to allow polar stratospheric clouds to form,” Deshler said.

That sort of information just isn’t possible to get using the sounding balloons the University of Wyoming team has launched each year from McMurdo Station for PSC measurements. Those balloons provide vertical profiles of particle size and number in a PSC as they soar to 35 kilometers and then parachute back to Earth.
“This information has helped [us] develop techniques to identify NAT within PSCs, but not to determine its formation temperatures, since these flights last only about three hours and do not follow air parcels,” Deshler said.
The instruments that will be aboard the balloons launched by the French space agency Centre National d’Etudes Spatiales (CNES) just outside McMurdo will remain aloft for several weeks or longer, and drift with the air. As the air cools, the instruments on board may observe the development of a PSC and thus the onset of NAT within the PSC.

“That’s the capability we’ve never had before: Fly at a reasonably constant altitude and constant density surface and remain aloft for weeks if not months,” said Deshler, a professor in the University of Wyoming’s Department of Atmospheric Sciences.

Finer details

Uncertainty also still exists on just how rapidly ozone loss occurs early in the process, according to Linnea Avallone, who is also using the CNES balloon platform to carry her instruments into the stratosphere.

The current computer simulations that model ozone loss do well enough over the length of the season, but don’t quite get it right in the beginning, Avallone explained. What’s happening in the early weeks of chemical havoc in the stratosphere will become increasingly important as the ozone hole heals. That’s because scientists’ ability to forecast ozone depletion will hinge on those details, she said.

“We’re hoping by making these measurements in a particular air mass that we’re following for a long time, we’ll be able to get some better constraints on why ozone is lost and the rate it’s lost,” said Avallone, an associate professor in the Department of Atmospheric and Oceanic Sciences at University of Colorado in Boulder.

Based on model predictions of the consequences of the Montreal Protocol, which barred the use of ozone depleting substances such as chlorofluorocarbons, the annual ozone hole over Antarctica is expected to “close” back to pre-1980s levels by mid-century.

Free-floating and free-falling

The ozone hole research is only part of the Concordiasi story.

About two-thirds of the balloons will carry instruments that will measure various atmospheric properties, such as temperature, pressure, humidity and winds in a region where such data are still very scarce, according to Stephen Cohn, a scientist at the National Center for Atmospheric Research (NCAR).

NCAR will fly an instrument package called a driftsonde on the balloon gondola that will release sensors, or dropsondes, that will parachute from about 20 kilometers to the ground in about 20 minutes. Each driftsonde can carry up to 50 dropsondes that it can drop by remote command.

“We get a full profile of the atmosphere from it,” Cohn explained. “It’s very different from a balloon flight where you’re only getting measurements from the flight level.”

The data from the dropsondes will help scientists tweak the data coming from satellites to ensure that what those orbiting eyes see in the lower atmosphere from outer space reflects reality.

“There’s really a dearth of observations at both poles. This is going to be a lot more measurements of what the atmosphere is really doing,” Cohn said.

Other techniques for measuring atmospheric properties include using GPS receivers on the gondolas, using a technique called radio occultation. Jennifer Haase at Purdue University has a Small Grant for Exploratory Research from the National Science Foundation for this part of the Concordiasi project.

As the line of sight of the GPS signal from any of the orbiting global positioning satellites passes deeper into the atmosphere, the signal path is refracted — bent and delayed — by the density changes in the atmosphere. The refraction is measured by the Doppler shift of the carrier frequency of the GPS signal, providing information on the pressure, temperature and humidity structure of the atmosphere.

The usefulness of the data from the dropsondes, GPS and other instruments goes beyond better operational weather forecasts for the next week. Rabier noted that eventually improving confidence in the use of satellite measurements will feed into climate models that account for long-term changes, such as the continental precipitation, that affect the growth and stability of Antarctica’s vast ice sheets.

“A critical issue for society is whether climate change can result in a significant change of the mass budget of the Antarctic ice sheet and, consequently, can impact global sea-level,” she said.

Antarctica holds about 70 percent of the world’s fresh water — enough to raise sea level by about 60 meters if it were possible to melt all at once, which would flood and devastate coastal regions around the world. While that scenario is unlikely, many scientists believe the sea level could rise by at least a meter by 2100. Antarctica will likely be an important part of that equation.

The technical details

This is not the first time CNES balloons have taken a spin around the atmosphere’s polar vortex above Antarctica.

In 2005, an international program called VORCORE launched similar long-duration balloons from McMurdo Station to study the dynamics of the polar vortex, a sort of stratospheric cyclone that forms during the spring in the Southern Hemisphere.

The French team investigated the vortex core mechanics to see how well it isolates the polar winter air. It is this isolation that allows the air to cool to PSC temperatures, and then for ozone to be destroyed by chlorine activated on PSCs during the winter.

The team launched 25 stratospheric balloons that averaged 58.5 days in the air, the longest flight lasting 109 days — enough time to allow the scientists to also investigate the dissipation of the polar vortex in late spring.

For that project, data were collected every 15 minutes. Concordiasi has seriously upgraded this capability to collect data at one-minute intervals. The longest flight should drift in the stratosphere for about four months.

Three test flights of the system and instruments — two dropsondes and one PSC instrument — were completed in February 2010 from the Republic of the Seychelles. All balloons stayed aloft well over a month. The PSC instrument stayed floated around for about two-and-a-half months, eventually circling the Earth, according to Deshler.

“It performed exceedingly well,” he said.

In order for the numerous U.S. and French sensors and instruments to work for months at a time, they must be able to sip slowly on the limited power available on the balloon gondola, and they must start at initial temperatures as cold as  minus 40 degrees centigrade, the design temperature for the gondola.

These proved to be very challenging aspects of the project for many of the groups.

“It was a pretty big instrument development activity,” Avallone said.

The breakthrough for the University of Colorado team was swapping the ozone photometers light source from power-hungry mercury lamps to a more efficient light-emitting diode (LED).

“It was a lot of laboratory and development work to make sure everything would function in the very severe temperature limits,” Avallone noted. “It’s not very common to be able to make a chemical measurement like this for just a couple of watts.”

Deshler said the University of Wyoming had similar technical challenges, forcing them to develop instrumentation that could start in the extremely low temperatures expected in the gondola. The energy-saving plan for Wyoming’s instrument will be to turn the instrument on for two of every 15 minutes when the balloon enters an area of cold temperatures at 20 kilometers altitude.

“There’s not enough power on the balloon gondola that we can operate continuously,” Deshler said. “We have to sample pretty deep into the cloud to see the formation of [the NAT] particles.”

Rabier said there was a major effort undertaken to use solar energy to power the experiments. “However, there are still some batteries for some of the instruments. Before dropping the sondes, there is a warm-up cycle to bring the instrument to a reasonable temperature before starting the measurements,” she explained.

Indeed, the polar spring, when darkness still prevails for much of the year, will present challenges for the solar-power-based instruments. But it’s the only time of the year that the researchers can make their measurements on the initial ozone depletion phase.

In addition, the polar vortex will be well formed at that time, meaning that for the first weeks of the program, the balloons will stay trapped over the southern pole area, with a potential to get a lot of dropsonde data in this area, Rabier said.

Still, the scientists expect a fast-paced schedule to complete all their plans.

“One of the challenges in the field will be to launch in a relatively short amount of time up to 18 balloons, when each flight preparation takes a few hours and the wind conditions are not always favorable,” Rabier said.

NSF-funded research in this story: Terry Deshler, University of Wyoming, Awards Nos. 0636946 and 0839124; Linnea Avallone, University of Colorado at Boulder, Award No. 0839017; Stephen Cohn and Terry Hock, National Center for Atmospheric Research, Award No. 0733007; and Jennifer Haase, Purdue University, Award No. 0814290.

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Curator: Peter Rejcek, Antarctic Support Contract | NSF Official: Winifred Reuning, Division of Polar Programs